1. Field of the Invention
The present invention relates to a polarizing element that can be used for display devices in various fields, such as personal computers, audio-visual equipment, mobile data communication devices, video games, simulation devices, car navigation systems and the like.
2. Description of the Related Art
Liquid crystal display devices that are used in displays for computers and the like modulate linearly polarized light and display an image. In conventional liquid crystal display devices, linearly polarized light was obtained by transmission of natural light at a polarizing plate formed of iodine-type elements or dichromatic-type elements. Hence, 50% of the natural light was absorbed and light utilization factors were low. This resulted in the problem of displays being dark. Moreover, the absorbed light energy was converted to heat energy, which could adversely affect the polarization characteristics of the polarizing plate. In light of these facts, it was hoped to improve luminance in liquid crystal display devices by, for example, using a prism sheet provided with a row of prisms. The prism sheet improved luminance by concentrating light paths to within an angle of visibility. However, the fact that any natural light having a particular polarization was absorbed at the polarizing plate could not be overcome thereby.
Consequently, various methods have been suggested to improve light utilization factors by splitting polarization components of natural light and then transmitting one component, reflecting another component and re-using the reflected light.
For example, WO92/22,938 (Japanese National Publication No. 6-508449) proposed a polarized beam splitting sheet formed by two prism sheets stuck together with prism surfaces facing each other. Each prism sheet was formed by a row of triangular prisms, and surfaces of the prisms were plurally laminated with thin films having mutually different refractive indexes (a large refractive index and a small refractive index). In this polarized beam splitting sheet, theoretically, natural light was incident at the Brewster angle. Accordingly, a p-polarization component was transmitted, and an s-polarization component was repeatedly reflected at thin film boundaries and re-used. Thus, the light utilization factor was improved. However, the thin films on the prism surfaces were formed using vapor deposition. Thus, production processes were complicated and production costs were extremely high. Because of these problems, this polarized beam splitting sheet was not implemented.
Further, WO95/17,303 Japanese National Publication No. 9-506837) has disclosed a polarized beam splitting sheet with multiply laminated oriented films having mutually different refractive indexes (a large refractive index and a small refractive index). This was marketed under the product name, xe2x80x9cDBEFxe2x80x9d. The polarized beam splitting sheet formed by this multi-layered film was effective in improving the light utilization factor. However, the sheet was formed by laminating several hundred thin film layers. Thus, production processes were complicated. Also, there was a problem in that, when film was being cut to a predetermined shape, offcuts generated large amounts of dust. In yet another technique, disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-281814, a cholesteric liquid crystal layer and a xc2xc-wave plate were combined. The cholesteric liquid crystal layer split two circularly polarized components and the xc2xc-wave plate converted circularly polarized light to linearly polarized light. This technique improved the light utilization factor. However, because a cholesteric liquid crystal layer and a xc2xc-wave plate were required, production costs were a problem.
The present invention is provided to solve the above-described conventional problems and an object of the present invention is to provide a polarizing element that can be produced at low cost and that can convert natural light to linearly polarized light with a high light utilization factor,
The above object of the present invention can be achieved by a polarizing element comprising a layer including liquid crystal molecules and a polarizer having a transmission axis, wherein the liquid crystal molecules have a chiral smectic texture of a helical configuration, the axis of the molecular helix of the helical configuration is along a direction other than a direction normal to a surface of the layer, and a direction of an orthogonal projection of the axis onto the surface of the layer is substantially at 90xc2x0 with respect to the transmission axis of the polarizer.
In one aspect of the polarizing element of the present invention, the layer has a chiral smectic C texture.
In another aspect of the polarizing element of the present invention, the chiral smectic texture is formed by liquid crystal molecules to be fixed one of physically and by a chemical reaction.
In another aspect of the polarizing element of the present invention, the chiral smectic texture of the helical configuration is formed by liquid crystal molecules to be fixed one of physically and by a chemical reaction.
In another aspect of the polarizing element of the present invention, the polarizer is one of an iodine-type polarizing plate, a dye-type polarizing plate and a polyvinylene-type polarizing plate, and the polarizer has a degree of polarization of at least 98%.
In another aspect of the polarizing element of the present invention, the layer is formed on a transparent substrate.
In another aspect of the polarizing element of the present invention, the transparent substrate is formed by one of a cellulose-type resin, a norbornene-type resin and a polycarbonate-type resin.
In another aspect of the polarizing element of the present invention, the transparent substrate also serves as a protective film for the polarizer.
In another aspect of the polarizing element of the present invention, of natural light that is incident from the direction normal to the surface of the layer, a linearly polarized light component whose vibration direction is substantially at 90xc2x0 to the orthogonal projection is transmitted and a vibration direction of a linearly polarized light component whose vibration direction is substantially parallel to the orthogonal projection is substantially altered by 90xc2x0 to be transmitted
In another aspect of the polarizing element of the present invention, the axis forms an oblique angle of from 5xc2x0 to 90xc2x0 with respect to the direction normal to the surface of the layer.
The above object of the present invention can be also achieved by a polarizing element comprising a layer including liquid crystal molecules and a polarizer having a transmission axis, wherein the liquid crystal molecules have a chiral smectic C texture of a helical configuration, the axis of the molecular helix of the helical configuration is along a direction other than a direction normal a surface of the layer, a direction of an orthogonal projection of the axis onto the surface of the layer is substantially at 90xc2x0 with respect to the transmission axis of the polarizer, and, of natural light that is incident from the direction normal to the surface of the layer, a linearly polarized light component whose vibration direction is substantially at 90xc2x0 to the orthogonal projection is transmitted and a vibration direction of a linearly polarized light component whose vibration direction is substantially parallel to the orthogonal projection is substantially altered by 90xc2x0 to be transmitted.
In one aspect of the polarizing element of the present invention, the chiral smectic C configuration is formed by liquid crystal molecules to be fixed one of physically and by a chemical reaction.
In the polarizing element of the present invention, natural light is incident from a direction normal to a layer formed of liquid crystal molecules. This liquid crystal layer has a specific chiral smectic texture of helical configuration, and particularly preferably has a chiral smectic C texture. The natural light can be converted to linearly polarized light by this structure. A conventional method for improving light utilization factors of polarizing elements is known in which, of two linearly polarized components included in natural light, one is transmitted and another is reflected, and, after being returned, a portion of the reflected light is added to transmitted light. A further method is known in which the two linearly polarized components of natural light are split and one component is transmitted through a xc2xd-wave plate so as to align vibration directions of the two components. That is, these conventional methods convert natural light to linearly polarized light by splitting the two linearly polarized components of the natural light and either returning one of the thus split components or using a xc2xd-wave plate. The polarizing element of the present invention is completely different from the conventional methods in that the polarizing element of the present invention can convert natural light to linearly polarized light in a single pass. Consequently, with the present invention, linearly polarized light can be obtained from natural light with a simpler architecture and higher efficiency. Moreover, when the present invention is used in a screen display or the like, both a great reduction in costs and an increase in display luminance can be achieved.
Furthermore, by combining the present invention with a conventionally used iodine-type, dye-type or polyvinylene-type polarizer, a polarizing element with a high luminance and a high degree of polarization can be formed. Therefore, when such polarizing elements are used for liquid crystal display elements, a high contrast display can be effectively obtained. In a method for combining the present invention with a conventional polarizer, an orthogonal projection of a chiral smectic axis is preferably substantially perpendicular to a transmission axis of the polarizer. Specifically, an angle between the orthogonal projection and the transmission axis is preferably from 60xc2x0 to 120xc2x0, more preferably from 80xc2x0 to 100xc2x0, and particularly preferably from 85xc2x0 to 95xc2x0.